This invention relates to autothermal reforming of steam and hydrocarbon to make syngas, used in manufacturing ammonia, methanol, Fisher-Tropsch synthesis, petroleum upgrading and other processes, and more particularly to autothermal reforming with recycle of a portion of the syngas to facilitate lower steam to carbon ratios without soot formation.
Autothermal steam reforming is well known and has been commercialized successfully. A mixture of steam and hydrocarbon is supplied to the autothermal reformer with air, oxygen-enriched air or oxygen and goes through partial combustion using a specially adapted burner at the top entry. The partial combustion products react on a fixed catalyst to form a syngas that usually includes steam, hydrogen, carbon monoxide and carbon dioxide. This process represents a fundamentally simple, reliable and cost effective technology for syngas production.
One operational characteristic desired for improvement, however, is that the autothermal reformer can also rely on external hydrogen supply for auto-ignition at start-up, e.g. 5 mole percent in the feed. Another characteristic is that a relatively high steam-carbon ratio is generally employed to ensure soot-free operation. High steam-carbon ratios can lead to increased capital costs since larger equipment is needed to heat and supply the feeds to the reformer, as well as to recover waste heat from the reformer effluent. High steam-carbon ratios are not attractive in modern megasyngas plants where minimized equipment sizes are needed to obtain a single-train process and economy of scale. Higher preheat temperatures for the feed mixture is also known to facilitate soot-free operation, but this can likewise be associated with high capital cost and energy consumption.
Recently, it has been suggested to add a pre-reformer in the feed stream of the steam-natural gas mixture to the autothermal reformer. This de-enriches the hydrocarbons and provides some hydrogen in the autothermal reformer feed, facilitating some reduction in the steam-carbon ratio. Still, more improvement in reducing the steam-carbon ratio is desired.
The present invention involves recycling a small portion of the autothermal reformer effluent into the steam-hydrocarbon feed stream, preferably with a thermo-compressor ejector that uses the preheated feed mixture as motive fluid. An ejector adapted for high temperature operation can achieve syngas recycle-motive fluid molar ratios from 0.2 to 1. The recycle gas flow rate is thus proportional to the hydrocarbon-steam feed mixture, which gives consistent, well-mixed hydrogen-steam enrichment at the outlet of the ejector. The exact ratio of recycle to motive fluid can be selected for specific applications to optimize the overall configuration.
The recycle introduces both hydrogen and steam, conveniently at an elevated temperature, into the feed to the autothermal reformer. The mixture leaving the ejector has a higher steam-carbon ratio, but also contains hydrogen from the recycle and has a higher feed temperature (where the recycle is at a higher temperature) so that the reformer can be operated in a soot-free regime to avoid plugging the catalyst bed and downstream equipment. There is a slight pressure drop between the raw feed steam-natural gas mixture and the reformer feed due to a loss across the ejector, which requires the raw feed mixture to be supplied at a slightly higher pressure, but this is offset by the lower pressure drop in the process heater and other upstream equipment due to lower quantities of steam in the front end, compared to the prior art without recycle. The process heater can also have a reduced duty, lowering capital costs and energy consumption. The downstream equipment can be reduced in size as well, since the proportion of steam in the reformer effluent is less, e.g. a smaller waste heat boiler and/or reforming exchanger can be used, and smaller other waste heat recovery and cooling equipment can be used. Meanwhile, the size of the autothermal reformer relative to total syngas product is about the same compared to the size needed for operation without effluent recycle.
In one embodiment, the present invention provides a steam reforming process including (a) heating a gas feed stream comprising a raw mixture of hydrocarbon and steam; (b) heating a second stream comprising oxidant gas; (c) supplying the heated gas feed stream in a feed line to an autothermal reformer with the heated second stream in an oxidant supply line; (d) recovering a syngas effluent stream from the autothermal reformer; (e) introducing a portion of the syngas effluent stream as recycle gas into the gas feed stream to obtain a feed mixture comprising hydrogen; (f) wherein a ratio of recycle gas to gas feed stream is from 0.2 to 1; and (g) operating the autothermal reformer at a steam to carbon ratio less than 3.6. The recycle gas is preferably introduced at a thermo-compressor ejector positioned in the feed line using the gas feed stream as motive fluid. The recycle gas is preferably at a higher temperature than the gas feed stream. The hydrocarbon is preferably natural gas. The oxidant gas can be selected from oxygen, oxygen-enriched air and air. The second stream can include steam. The feed mixture comprises from 5 to 50 mole percent hydrogen. The raw mixture preferably has a steam to carbon ratio from 0.6 to 3. The steam reforming process can also include cooling the syngas effluent stream and taking the recycle gas from the cooled syngas effluent stream. The syngas effluent stream can be cooled in a waste heat boiler or a reforming exchanger. Preferably, the ratio of recycle gas to motive fluid is from 0.3 to 0.7 and the feed mixture comprises from 20 to 40 mole percent hydrogen. The method can also include pre-reforming gas feed stream upstream from the ejector, preferably before the heating of the gas feed stream.
In another embodiment, the present invention provides a steam reforming process that includes (a) heating first and second gas feed streams comprising a raw mixture of hydrocarbon and steam; (b) heating a third stream comprising oxidant gas; (c) supplying the first heated gas feed stream in a feed line to an autothermal reformer with the third stream in an oxidant supply line; (d) recovering a first syngas effluent stream from the autothermal reformer; (e) supplying the second heated stream to a reforming exchanger for endothermic catalytic conversion in tubes in the reforming exchanger to form a second syngas effluent stream; (f) mixing the first syngas effluent with the second syngas effluent to form a syngas mixture; (g) passing the syngas mixture across the tubes in the reforming exchanger in heat exchange therewith to supply a cooled syngas product to a syngas product line; (h) introducing a portion of the syngas product as recycle gas into the first gas feed stream to obtain a feed mixture comprising hydrogen, wherein a ratio of recycle gas to first gas feed stream is from 0.2 to 1; and (i) operating the autothermal reformer at a steam to carbon ratio less than 3.6. This is possible in this embodiment due to hydrogen and steam enrichment, as well as any increase in the feed temperature.
In this embodiment, the recycle gas is preferably introduced at a thermo-compressor ejector positioned in the feed line using the gas feed stream as motive fluid. The recycle gas is preferably at a higher temperature than the gas feed stream. The hydrocarbon is preferably natural gas. The oxidant gas can be selected from oxygen, oxygen-enriched air and air. The third stream can include steam. The feed mixture preferably comprises from 5 to 50 mole percent hydrogen. The raw mixture preferably has a steam to carbon ratio from 0.6 to 3. Preferably, the ratio of recycle gas to the first gas feed stream is from 0.3 to 0.7 and the feed mixture comprises from 20 to 40 mole percent hydrogen. The method can also include pre-reforming gas feed stream upstream from the ejector, preferably before the heating of the gas feed stream.
In a further embodiment, the invention provides apparatus for steam reforming. The apparatus includes means for heating a gas feed stream comprising a raw mixture of hydrocarbon and steam and means for heating a second stream comprising oxidant gas. Means are provided for supplying the heated gas feed stream in a feed line to an autothermal reformer with the second stream in an oxidant supply line. Means are provided for recovering a syngas effluent stream from the autothermal reformer. Means are provided for introducing a portion of the syngas effluent stream as recycle gas into the gas feed stream at a thermo-compressor ejector positioned in the feed line using the gas feed stream as motive fluid to obtain a feed mixture comprising hydrogen, wherein a ratio of recycle gas to motive fluid is from 0.2 to 1. Means are provided for operating the autothermal reformer at a steam to carbon ratio less than 3.6. The invention can also include a pre-reformer for pre-reforming the gas feed stream upstream from the ejector, preferably before the heating of the gas feed stream.
In a further embodiment, the present invention provides apparatus for steam reforming that includes means for heating first and second gas feed streams comprising a raw mixture of hydrocarbon and steam, means for heating a third stream comprising oxidant gas, means for supplying the first heated gas feed stream in a feed line to an autothermal reformer with the third stream in an oxidant supply line, means for recovering a first syngas effluent stream from the autothermal reformer, means for supplying the second heated stream to a reforming exchanger for endothermic catalytic conversion in tubes in the reforming exchanger to form a second syngas effluent stream, means for mixing the first syngas effluent with the second syngas effluent to form a syngas mixture, means for passing the syngas mixture across the tubes in the reforming exchanger in heat exchange therewith to supply a cooled syngas product to a syngas product line, means for introducing a portion of the syngas product as recycle gas into the first gas feed stream at a thermo-compressor ejector positioned in the feed line using the first gas feed stream as motive fluid to obtain a feed mixture comprising hydrogen, wherein a ratio of recycle gas to motive fluid is from 0.2 to 1, and means for operating the autothermal reformer at a steam to carbon ratio less than 3.6. The invention can also include a pre-reformer for pre-reforming the gas feed stream upstream from the ejector, preferably before the means for heating of the gas feed stream.
A still further embodiment of the invention provides a method for starting up the apparatus just described, for continuous operation. The method includes the steps of: (a) heating the first and second gas feed streams before starting the third stream, wherein the first and second feed streams are essentially free of added hydrogen; (b) introducing a hydrogen-generating compound into the first stream, second stream, or combination thereof, that is decomposed in the autothermal reformer, reforming exchanger, or combination thereof, respectively, to form hydrogen gas; (c) recycling the syngas product from the reforming exchanger into the first gas feed stream; (d) when the first gas feed stream reaches or exceeds its minimum auto-ignition temperature at the autothermal reformer inlet, starting the third stream to obtain auto-ignition in the autothermal reformer; and (e) after the auto-ignition is established, terminating step (b).